Toxoplasma gondii is a ubiquitous Apicomplexan protozoan parasite of mammals and birds. It has long been recognized as an important opportunistic pathogen of immune compromised hosts and emerged as a major opportunistic pathogen of the Acquired Immunodeficiency Syndrome (AIDS) epidemic. The predilection of this parasite for the central nervous system (CNS) causing necrotizing encephalitis constitutes its major threat to patients with AIDS. In immune compromised patients, such as those with AIDS, it is believed that CNS toxoplasmosis is due to the reactivation of a chronic or latent infection with tissue cysts (bradyzoites) of T. gondii. In prior studies we have established that bradyzoites within tissue cysts express unique proteins that facilitate their survival. The cyst wall is a critical structure for the survival, reactivation and oral transmission of T. gondii. The widespread distribution of T. gondii in humans and other animals is due to the ability of tissue cysts to permit oral transmission of this infection. In the present proposal, studies will be focused on the characterization of the components of the cyst wall. This will included further characterization of CST1 (a glycoprotein that binds DBA), which we had previously identified as being a cyst wall component that is expressed rapidly during the differentiation process. The components of the cyst wall will be characterized using a combination of proteomic, immunologic and genetic approaches taking advantage of techniques we have developed for purification of the T. gondii cyst wall. These studies will both elucidate the composition of this structure and provide reagents that should prove useful for further studies of the biology and organization of the cyst wall. Defining the structure of the cyst wall is clearly a critical step for understanding the biology of Toxoplasma gondii and should provide valuable information for the development of new strategies for the prevention of reactivation toxoplasmosis.